Viscosity is a measure of resistance to flow for a liquid. Higher viscosity means a stronger resistance to flow. There are two types of viscosity measurements. The first is dynamic viscosity, which is a measure of the internal resistance to flow of a liquid. The second is kinematic viscosity, which is the liquid's resistance of flow under the force of gravity. Kinematic viscosity is defined as dynamic viscosity divided by density of the liquid.
Viscosity is an important property for liquid, especially for petroleum liquids such as fuels and lubricants and is often specified in the trading of petroleum products. Also, the manner in which the measurement is conducted is specified in test standards such as the ASTM D445 standard. Inherent to the ASTM D445 standard is an expectation of precision. A slight deterioration in precision can have a profound impact on the cost of producing petroleum products, especially lubricants. For example, a one percent (1%) error in the measurement of kinematic viscosity can lead to a one million dollar per year of loss revenue for a large lubricant blending plant. Thus, it is well appreciated in the industry that tight control on the measurement precision of viscosity is critical to the profitability of manufacturing plants.
ASTM D445 specifies the apparatus and procedures for measuring the kinematic viscosity of petroleum liquids. It involves the use of a glass capillary system. In essence, a fixed amount of liquid sample is allowed to flow under gravity through a glass viscometer submerged in a constant temperature liquid bath, and the time taken to flow between timing marks is used to calculate the resultant viscosity.
There are numerous challenges in practicing the ASTM D445 standard. The apparatus is typically large, heavy, and difficult to transport. While a glass capillary column is only about one foot in length itself, the entire system incorporating a liquid bath can be substantially larger. The capillary must be immersed in a constant temperature liquid bath, typically set between forty and one-hundred degrees Celsius. The liquid bath wall is made of glass to allow visual observation of the meniscus of the sample liquid.
Apart from taking up a lot of space in the laboratory, the glass capillary is fragile, and because of its often elaborate construction in accommodating various bends and restrictions in the flow path, difficult to clean. Cleaning is a critical part of the viscosity measurement procedure because a slight contamination of the sample by residuals from the previous sample or cleaning agent could have a significant impact on subsequent viscosity measurement results. Cleaning is also the most time-consuming part of the whole testing process, as it requires multiple rinsing of the capillary with solvents followed by drying between each rinsing. Viscous samples such as lubricants and heavy fuel oils are more difficult to clean than less viscous materials such as jet fuels and diesels. Moreover, the liquid bath is a potential fire and burn hazard since the glass wall is vulnerable to breakage with its large volume of hot liquid when operated at high temperatures. There are also technical challenges in maintaining stable temperature in the liquid bath and in relying on human operators to precisely measure the flow time of liquid in the capillary using stopwatches.
There have been attempts by different systems to overcome the disadvantages of the ASTM D445 standard. Most of these attempts have not received universal acceptance due to their own unique disadvantages, such as a lack of precision in the test results; or mechanical complexity of the machines required to complete the test. Most of them solely rely on gravity to drive the flow of the liquid column. They do not include any provision for cleaning the capillary tube at the end of a test. In fact, a lot of them promote the idea of using a disposable tube. To achieve the precision of less than one percent (1%) typically required of measured value for petroleum products, each capillary tube must be carefully calibrated. Calibration means that the tube must be tested repeatedly for multiple standard samples. The tube must be clean prior to each test to avoid contamination from previous sample(s). Therefore cleaning is a very important aspect for a calibrated viscometer. Some of the system's measurement of flow time relies on the detection of the trailing meniscus at the end of the liquid column. This will not be suitable for opaque samples which occur frequently in petroleum samples. Opaque sample can soil, stain, or otherwise leave a smear on the walls of the tubing, rendering detection of the passage of the trailing meniscus unclear or even impossible to observe.
It is an object of the present invention to provide a compact device that is capable of determining viscosity of liquids in a precise, yet convenient manner.
It is an object of the present invention to provide a compact device that reduces cleaning and cleaning time.